The lowest end FirePros are what I went for (reference V3900/V4900): then I strapped an Accelero S1 Plus on them.
Although I've built my own PCs, I've previously been nervous of modding a graphics card, but I've had a look at the V4900 and the Accelero S1 Plus, and this combination certainly looks to be an excellent suggestion. I'm particularly impressed by the KitGuru
review of the Accelero S1 Plus.
I don't know if there are different implementations of the V4900, but would the Sapphire AMD FirePro V4900